This invention relates to chemical vapor deposition methods and apparatus therefor. More particularly, the invention relates to such methods and apparatus using a liquid source from which a source gas is generated.
In the past, source gases were used in most chemical vapor deposition methods because they are easy to handle and their flow can be controlled accurately. Methods of chemical vapor deposition by vaporizing a liquid source to generate a source gas, by contrast, have not been popular because it has not been easy to accurately supply a specified amount of the liquid source or to control the temperature of the supply piping. If it becomes possible to more precisely control the flow of a liquid source and hence to easily supply a source gas, however, the choice of available source materials will increase significantly and this may provide a partial answer to the ever increasing need for a method of forming thin films in the manufacture of finer and more highly integrated semiconductor devices.
FIG. 8 shows schematically a prior art device for supplying a liquid source, characterized as having a source tank 92 and a gas mass flow controller (GMFC) 94 enclosed inside a thermostatic chamber 90, the gas mass flow controller 94 being connected to a process chamber 96 to which is also connected a vacuum pump 98. After a liquid source (L) is placed inside the source tank 92, it is heated by a heater 93 to generator a vapor and the generated vapor is supplied into the process chamber 96 with its flow rate controlled by the gas mass flow controller 94.
A conventional device of this type has many disadvantages. Firstly, the device becomes large because many of its components must be enclosed inside a thermostatic chamber. This also tends to increase the cost of the device. Secondly, since the flow of a source gas evaporated by the heat from the heater must be controlled, and since a relatively large amount of high-temperature gas must be controlled by the gas mass flow controller, the device becomes large and expensive and its reliability is adversely affected as compared to the control of a liquid. Thirdly, thermal decomposition of the liquid source becomes likely when a high-temperature vapor is generated therefrom. Fourthly, since a vapor is generated in the source tank and the source gas thus generated is carried into the process chamber through the gas mass flow controller, the total gas flow path becomes inconveniently long. As shown in FIG. 9 and will be explained in detail below, this tends to increase the time for stabilizing the flow rate when the supply of the source is started (start-up) and when it is stopped (shut-down).